The increasing concentration of population in urban settings presents important environmental problems. Prominent among them is presented when it becomes desirable, from a public health or aesthetic consideration, to remove unwanted odorous constituents from a gas stream, prior to its release into the atmosphere.
Various techniques have been developed in response to the need for odorous constituent removal. As a general rule, an effective technique should be tailored to the particular constituent to be removed. In the waste water system environment, a primary cause of odor is hydrogen sulfide which is detectable by the human olfactory sense at very low concentrations. In addition to the unpleasant odor associated with it, hydrogen sulfide is noted for its toxicity and its capacity for corroding materials with which it comes in contact.
Because of the undesirability of introducing hydrogen sulfide into the atmosphere, it is desirable to have systems for removing, in a cost effective manner, substantial amounts of the unwanted substance, preferably at or near the source of its production.
As more particularly discussed in copending U.S. patent application Ser. No. 08/427,128, removal of odorous constituents from a waste gas stream can be accomplished by several techniques. In many cases, an odor control technique of choice is absorption, or chemical oxidation, a process in which odorous constituents in a gas stream are transferred into a liquid solution and are chemically oxidized in the liquid phase. Acceptance of this process is due, at least in part, to cost considerations, especially when large gas volumes, having relatively low concentrations of odorous constituents, are involved. In general, the technique is utilized in a packed tower conventionally known as a scrubber.
In scrubber systems utilizing the absorption method, reaction between chemical treating agents and the odorous constituents takes place in the liquid phase. Removal efficiency depends on the transfer rate of the compounds from air into liquid, and reaction kinetics. This, in turn, is dependent on the mass transfer coefficient and total interfacial surface area. Such considerations drive scrubber packed tower design.
A significant problem in conventional packed towers is plugging of the packing as a result of accumulation of solids. Such accumulations can quickly lead to reduced system efficiency. Plugging may be caused by a variety of operational parameters such as hardness of make-up water, the chemical reagents utilized, and system pH. It is generally recognized that plugging can be effectively reduced or eliminated by purging the system. In the purging process, an amount of the recirculation stream, together with an equal or greater amount of the by-product salts created and added to the system, must be constantly removed from the sump to prevent the accumulation of solids and resultant plugging. Fresh make-up water and new chemical reagent must be added to replace the purge stream.
However, a major concern in consideration of system purge rate is the cost of unreacted chemical reagent which is sent to the drain during the purging process. Over time, this cost can make system operation prohibitively expensive. The alternative, an inefficient, plugged system, is also unacceptable.
In other cases, hardness in the water supply and the carbonates which are often present cause precipitates to form on packing medium surfaces, especially when the system is operated at an elevated pH. In some cases, it becomes necessary to shut down the odor control system in order to acid wash or otherwise clean control system surfaces.
In view of the foregoing, it would be highly desirable to have an odor control system which could function efficiently while reducing scaling problems. Of course, it would be beneficial if such a system would conserve reagents, thereby reducing operating costs.
A review of prior art systems reveals that one conventional technique for scrubbing hydrogen sulfide includes using only sodium hydroxide while, in another technique, sodium hydroxide and sodium hypochlorite are utilized to oxidize the hydrogen sulfide. The advantages and disadvantages of each technique deserve some mention.
In the first case, in which NaOH only is used, typically in a single stage system, a reaction occurs as follows: EQU H.sub.2 S+2NaOH.fwdarw.Na.sub.2 S+2H.sub.2 O
An advantage of this conventional technique is that it is relatively inexpensive to operate since the cost to remove H.sub.2 S with sodium hydroxide is about eight to ten times less expensive than with sodium hypochlorite. As an example, for a gas stream flowing at 10,000 cfm, bearing 100 ppm H.sub.2 S, to have about 127 lbs./day of H.sub.2 S removed, the annual cost with NaOH as the reagent of choice is about $20,000 per year, compared with about $293,000 with other conventional systems, as pointed out below.
However, the cost advantage of the sodium hydroxide process is offset by several limitations. One limitation of the sodium hydroxide technique is that scaling is a problem. Another is that a long residence time is required for thorough gas treatment. This means that facility costs are increased because a large treatment tower is required. Another significant limitation is evident in some cases when system blow down is discharged into a sewer having an acidic pH. In these cases, the above reaction is reversible and, when the pH drops below about 7, H.sub.2 S is regenerated.
In addition to the foregoing limitations in the conventional sodium hydroxide system, such systems operate at high pH (typically 11-12). Hydrogen sulfide containing gases typically contain carbon dioxide which, at elevated pH, reacts with sodium hydroxide as follows: EQU CO.sub.2 +2NaOH.fwdarw.Na.sub.2 CO.sub.3 +H.sub.2 O
Thus, it will be appreciated that the presence of the carbon dioxide, coupled with the requirement of operating at an elevated pH, results in the need for additional sodium hydroxide than would be predicted for hydrogen sulfide removal alone. This factor, of course, means increased reagent usage resulting in elevated operating costs over the life of the system.
In the second conventional technique for removing hydrogen sulfide from a gas stream, sodium hydroxide is utilized in combination with sodium hypochlorite. System reactions, assuming complete oxidation, are as follows: EQU H.sub.2 S+2NaOH+4NaOCl.fwdarw.Na.sub.2 SO.sub.4 +4NaCl+2H.sub.2 O EQU NaOCl+H.sub.2 S.fwdarw.NaCl+H.sub.2 O+S
Advantages of this process, in contrast to the use of sodium hydroxide alone, are that the system pH is generally lower (typically 9-10) and CO.sub.2 absorption is reduced. In addition, less scaling occurs and there is less H.sub.2 S regeneration. Further, a smaller scrubber tower may be utilized, thereby reducing facility operating costs.
A significant disadvantage of the sodium hydroxide/sodium hypochlorite process is reagent cost. For example, in scrubbing the 10,000 cfm gas stream mentioned above, from which 127 lbs./day of H.sub.2 S are removed, typical annual operating costs can be as high as $293,000.
By way of summary of advantages and disadvantages of the two conventional processes described herein, reference may be made to Table I.
TABLE I ______________________________________ COMPARISON OF SYSTEMS UTILIZING NaOH WITH SYSTEMS UTILIZING NaOH and NaOCl ______________________________________ NaOH ADVANTAGE DISADVANTAGES ______________________________________ Low Operating Cost High pH (11-12) Scaling CO.sub.2 Absorption H.sub.2 S Regeneration Longer Residence Time ______________________________________ NaOH and NaOCl ADVANTAGES DISADVANTAGE ______________________________________ Lower pH (.about.10) High Operating Cost Lower CO.sub.2 Absorption Less Scaling No H.sub.2 S Regeneration Smaller Tower ______________________________________
As Table I shows, there are several advantages to the second process. In view of the comparative results, the second process would be expected to be the system of choice in many cases. However, the high annual operating cost, multiplied by the many years of useful life of a scrubber using the process, often makes the process economically prohibitive.
In view of the foregoing, it would be desirable to have an odor control system which affords the advantages of conventional systems while substantially avoiding their limitations. Ideally, such a system would be capable of efficient performance over many years with substantial reductions in operating costs, as compared to conventional systems.